I had a rectangle piece of fabric. The kids pointed out how big they wanted it (and I checked to make sure it was long enough for an unsharpened pencei) and I trimmed the fabric to match their requests. I did double over the fabric to make it heavier and stronger. Sewed in the zipper to connect the ends. Folded it inside out so the zipper was in the middle. Sewed up the side seams. Wacked off the extra length of zipper. Sewed the corners at a diagonal to make the depth. Done.
My kids (boy and girl 9 year old twins) were just showing their new cases to their friends that stopped by. Now they each want one. I told them to come back tomorrow and we'll get out the paint and fabric and they can go at it. A couple of years ago, I bought a humongous box of zippers on ebay--I use them for EVERYthing.